I have never heard this, before. FWIW, my '87 is twin plug and was equipped with the air injection (now removed and plugged). When I was shopping around for a machine shop to handle my parts, not one of them mentioned this issue, either - I dealt with some pretty well respected shops, too.

As for the second set of spark plugs, your lower valve covers are machined (holes drilled) for the plugs to be inserted into their respective places. The only plug that I have accessibility problems with is my lower number three, as the valve cover hole isn't very large (looks like a half moon) and the turbo scavenge oil pump butts up against the cover. No biggie, I just remove the valve cover if I never need to get that one.

I can post some pictures of the twin plug process if need be...

__________________
-jeff
back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction..."Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)